In many integrated circuits the effects of distributed capacitance between a thin film resistor and the underlying silicon substrate are not a critical factor in achieving desired circuit performance because the thin film resistor is not physically so large that an RC time constant formed by its resistance and associated parasitic capacitance introduces signal phase shifts that prevent desired circuit performance from being achieved. Thin film resistors and diffused resistors having resistances of over about 100 kilohms consume a great deal of surface area of an integrated circuit. Consequently, integrated circuit designers traditionally have avoided design of circuits requiring such large resistances. For example, use of physically large thin film resistors (with large associated parasitic capacitances) ordinarily would be avoided in an amplifier feedback path wherein substantial phase shifts would be expected to produce significant design problems, such as lowering gain margin and phase margin, possibly causing significant signal overshoot and "ringing", and possibly even resulting in sustained undesired oscillations, (which occur if the amplifier gain exceeds unity when the phase shift is 180 degrees). Other reasons for avoiding physically large resistors in an integrated circuit include a disproportionately large increase in chip cost that results when physically large resistors are to be included on the chip.
There are many electronic applications for amplifiers in which the amplifier input must be "isolated" from the amplifier output so that large common mode input signals (of several hundred volts to several thousand volts) do not produce a corresponding common mode output voltage; that is, only the incremental difference between the input terminals of the amplifier produces a corresponding difference in the voltage between the amplifier output terminals. Up to now, such so called "isolation amplifiers" have been almost universally used for such applications. Isolation amplifiers include "isolation barriers", such as isolation transformers, capacitive coupling, or optical coupling to provide "galvanic isolation" between the output terminal and the input terminals of the isolation amplifier, so that there is no DC path between the inputs and any output of the isolation amplifier. Unfortunately, isolation amplifiers are inherently expensive, because separate isolated power supplies are required for the input and output portions of the isolation amplifier on opposite sides of the isolation barrier. Isolation amplifiers are incapable of achieving nearly as good levels of gain accuracy, input offset voltage drift, linearity, and bandwidth as an amplifier not having a galvanic isolation barrier.
However, there appear to be many electronic applications for an amplifier in which a high degree of isolation between the amplifier inputs and outputs is needed, but pure galvanic isolation is unnecessary, as long as the amplifier can accept high common mode signals of .+-.200 volts operating on standard .+-.15 volt power supplies. However, up to now, no one as been able to design such an amplifier perhaps because integrated circuit designers traditionally avoid use of physically large integrated circuit resistors. Very large voltages applied across resistors result in increased power dissipation (which is proportional to the square of the voltage applied across a resistor). Substantial circuit design problems, including increased noise, large distributed capacitance associated with the resistors resulting in poor AC performance, poor common mode rejection that would be expected from difficulties in precisely matching physically large resistors, and serious thermal matching problems all probably have led those skilled in the art to avoid attempts to produce an integrated circuit difference amplifier that might be useable in many applications rather than an isolation amplifier.